1. Field of the Invention
This invention relates to cutter assemblies for rolling cone earth boring bits, specifically to the hard inserts for use in such cutter assemblies.
2. Summary of the Prior Art
Earth-boring bits of the rolling cone variety rely on the rolling movement of at least one cutter over the bottom of the bore hole for achieving drilling progress. The earth-disintegrating action of the rolling cone cutter is enhanced by providing the cutter with a plurality of protrusions or teeth. These teeth are generally of two types: milled teeth, formed from the material of the rolling cone; and inserts, formed of a hard material and attached to the rolling cone surface.
One measure of a rolling cone earth-boring bit's performance is its ability to "hold gage," or maintain a consistent borehole diameter over the depth or length of the borehole. Maintenance of a consistent borehole diameter expedites and simplifies the drilling process because drill strings may be removed from and inserted into a hole of generally consistent diameter more easily than a borehole of varying diameter. Gage holding ability is of particular importance in directional drilling applications.
To achieve this gage holding ability, the rolling cones of such earth boring bits have been provided with hard inserts on the outermost, or gage, surface of the rolling cones. These gage inserts have functioned primarily as wear pads that prevent the erosion of the gage surface of the rolling cone, thereby permitting the earth boring bit to hold a more consistent gage or borehole diameter. One example of such an insert is disclosed in U.S. Pat. No. 2,774,571, Dec. 18, 1956, to Morlan. Other gage inserts are shown in U.S. Pat. No. 3,137,335, Jun. 16, 1964, to Schumacher; U.S. Pat. No. 3,389,761, Jun. 25, 1968, to Ott; and U.S. Pat. No. 4,729,440, Mar. 8, 1988, to Hall.
Two staggered rows of such gage inserts are disclosed in U.S. Pat. No. 4,343,372, Aug. 10, 1982, to Kinzer. U.S. Pat. No. 4,940,099, Jul. 10, 1990, to Deane et al., discloses alternating polycrystalline diamond and tungsten carbide gage inserts mounted substantially flush with the gage surface of the rolling cone cutter.
The gage inserts described in the above references are passive in operation, that is, they serve only as wear-resistant inserts and are not designed to actively cut the gage of the borehole. Such wear-resistant inserts are susceptible to heat-cracking and spalling in operation, and may fail to provide adequate gage-holding ability. Loss of gage-holding ability or gage protection can lead to lower rates of penetration and decreased seal and bearing life.
A Smith International, Inc. promotional brochure entitled "Smith Steerable-Motor Bits On Target For Your Drilling Program" discloses chisel-shaped inserts on the gage surface that protrude a great distance from the gage surface. It is believed that these inserts may be easily broken due to bending stress present in the inserts because of their extreme protrusion. It is further believed that rounded cutting edges associated with chisel-shaped inserts are susceptible to heat-cracking and spalling similar to passive wear-resistant inserts. Chisel-shaped inserts also provide less wear-resistance than flat-tipped inserts because only the rounded chisel crest is in tangential contact with the wall of the borehole.